Subterranean treatment fluids, friction reducing copolymers, and associated methods

ABSTRACT

Methods of treating a portion of a subterranean formation, comprising: providing an aqueous treatment fluid comprising water, and a friction reducing copolymer that comprises acrylamide and an acrylic acid ester; and introducing the aqueous treatment fluid into the portion of the subterranean formation. Methods of fracturing a subterranean formation comprising: providing an aqueous treatment fluid comprising water, and a friction reducing copolymer that comprises acrylamide and an acrylic acid ester; and introducing the aqueous treatment fluid into the subterranean formation at or above a pressure sufficient to create one or more fractures in the subterranean formation. Aqueous treatment fluids comprising: an aqueous fluid and a friction reducing copolymer that comprises acrylamide and an acrylic acid ester.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is related to U.S. patent application Ser. No.11/504,938, entitled “Subterranean Treatment Fluids, Friction ReducingCopolymers, and Associated Methods,” filed on the same date herewith,the entire disclosure of which is incorporated herein by reference.

BACKGROUND

The present invention relates to subterranean treatments, and moreparticularly, to aqueous subterranean treatment fluids that comprisefriction reducing copolymers and associated methods.

Aqueous treatment fluids may be used in a variety of subterraneantreatments. Such treatments include, but are not limited to, stimulationoperations and completion operations. As used herein, the term“treatment,” or “treating,” refers to any subterranean operation thatuses a fluid in conjunction with a desired function and/or for a desiredpurpose. The term “treatment,” or “treating,” does not imply anyparticular action by the fluid.

An example of a subterranean treatment utilizing an aqueous treatmentfluid is hydraulic fracturing. Hydraulic fracturing is a processcommonly used to increase the flow of desirable fluids, such as oil andgas, from a portion of a subterranean formation. In hydraulicfracturing, a fracturing fluid is introduced into the subterraneanformation at or above a pressure sufficient to create or enhance one ormore fractures therein. Enhancing a fracture includes enlarging apre-existing fracture in the formation. One type of hydraulic fracturingis commonly referred to as “high-rate water fracturing.” Typically,high-rate water fracturing is utilized in subterranean formations withlow permeability (e.g., no more than about 0.1 millidarcy). Unlikeconventional fracturing fluids, fluids used in high-rate waterfracturing generally do not contain a sufficient amount of polymer toform a gel. Gel formation is based on a number of factors including theparticular polymer and concentration thereof, temperature, and a varietyof other factors known to those of ordinary skill in the art. As aresult, the fracturing fluids used in high-rate water fracturingoperations generally have a lower viscosity than traditional fracturingfluids.

During the placement of aqueous treatment fluids into a well bore, aconsiderable amount of energy may be lost due to friction between thetreatment fluid in turbulent flow and the formation and/or tubular goods(e.g., pipes, coiled tubing, etc.) disposed within the well bore. As aresult of these energy losses, additional horsepower may be necessary toachieve the desired treatment. To reduce these energy losses, frictionreducing polymers have heretofore been included in aqueous treatmentfluids. The term “friction reducing polymer,” as used herein, refers toa polymer that reduces frictional losses due to friction between anaqueous fluid in turbulent flow and tubular goods (e.g. pipes, coiledtubing, etc.) and/or the formation. These friction reducing polymers maybe synthetic polymers, natural polymers, or viscoelastic surfactants andare thought to reduce the friction between the aqueous treatment fluidin turbulent flow and the tubular goods and/or the formation.

In some instances, friction reducing polymers are oil-externalemulsions, wherein upon addition to the aqueous treatment fluid, theemulsion should invert releasing the friction reducing polymer into thefluid. One such friction reducing polymer is a copolymer of acrylic acidand acrylamide. However, it is believed that the ionic nature of certainfriction reducing polymers (such as the aforementioned copolymer) maycause these polymers to interact with formation fines and/or salts,particularly polyvalent metal cations, and thereby form flocs. The term“floc” as used herein, refers to a coagulated mass of particles in aliquid. The resulting flocs may be undesirable because, among otherthings, the flocs may facilitate the formation of agglomerates that mayclog pumps, filters, surface equipment and possibly plug fractures.Similarly, flocs may also reduce the fluid conductivity in the formationby adsorbing onto fracture faces within the formation or by possiblyforming a stable emulsion in the formation that impacts subsequentproduction from the well bore.

SUMMARY

The present invention relates to subterranean treatments, and moreparticularly, to aqueous subterranean treatment fluids that comprisefriction reducing copolymers and associated methods.

One embodiment of the present invention is a method of treating aportion of a subterranean formation, comprising: providing an aqueoustreatment fluid comprising water, and a friction reducing copolymer thatcomprises acrylamide and an acrylic acid ester; and introducing theaqueous treatment fluid into the portion of the subterranean formation.

Another embodiment of the present invention is a method of treating aportion of a subterranean formation, comprising: providing an aqueoustreatment fluid comprising water, and a friction reducing copolymer thatcomprises acrylamide in an amount in the range of from about 60% toabout 95% by weight and an acrylic acid ester in an amount in the rangeof from about 5% to about 40% by weight; and introducing the aqueoustreatment fluid into the portion of the subterranean formation.

Yet another embodiment of the present invention is a method offracturing a subterranean formation comprising: providing an aqueoustreatment fluid comprising water, and a friction reducing copolymer thatcomprises acrylamide and an acrylic acid ester; and introducing theaqueous treatment fluid into the subterranean formation at or above apressure sufficient to create one or more fractures in the subterraneanformation.

The features and advantages of the present invention will be apparent tothose skilled in the art. While numerous changes may be made by thoseskilled in the art, such changes are within the spirit of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to subterranean treatments, and moreparticularly, to aqueous subterranean treatment fluids that comprisefriction reducing copolymers and associated methods.

I. Aqueous Treatment Fluids of the Present Invention

The aqueous treatment fluids of the present invention generally comprisean aqueous fluid, and a friction reducing copolymer of the presentinvention that comprises acrylamide and an acrylic acid ester. The term“friction reducing copolymer,” as used herein, refers to a copolymerthat reduces frictional losses due to friction between an aqueoustreatment fluid in turbulent flow and tubular goods (e.g. pipes, coiledtubing, etc.) and/or the formation. The term “copolymer,” as usedherein, is not limited to polymers comprising two types of monomericunits, but includes any combination of monomeric units, e.g.,terpolymers, tetrapolymers, and the like.

Generally, a friction reducing copolymer of the present invention may beincluded in any aqueous treatment fluid used in subterranean treatmentsto reduce friction. Such subterranean treatments include, but are notlimited to, stimulation treatments (e.g., fracturing treatments,acidizing treatments, fracture acidizing treatments) and completionoperations. Those of ordinary skill in the art, with the benefit of thisdisclosure, will be able to recognize a suitable subterranean treatmentwhere friction reduction may be desired.

Suitable aqueous fluids that may be used in the present inventioninclude fresh water, salt water, brine, seawater, or combinationsthereof. Generally, the aqueous fluid used may be from any source,provided it does not contain an excess of compounds that may adverselyaffect the other components used in accordance with this invention orthe subterranean formation. While the friction reducers of the presentinvention may be suitable for use in a variety of aqueous treatmentfluids, they may be particularly useful in treatment fluids wherein afriction reducing polymers' reduced sensitivity to salt is desired.

The friction reducing copolymers of the present invention compriseacrylamide and an acrylic acid ester. Generally, the acrylic acid esterpresent in the friction reducing copolymers of the present invention maybe any acrylic acid ester that maximizes friction reduction whileminimizing flocculation and salt intolerance. In determining a suitableacrylic acid ester for use in the present invention, a variety oftechniques may be used including, but not limited to, determining theradius of gyration for a particular friction reducing copolymer in thepresence of interfering salts. Generally, including an acrylic acidester that will give the copolymer a larger radius of gyration isdesirable. It is believed that friction reducing polymers possess largeradii of gyration, in addition to generally having a molecular weightgreater than 7,500,000 atomic mass units (“amu”). While removing theanionic character of these polymers may help with preventing theformation of flocs, it is likely that charge repulsion helps increasethe radius of gyration. The acrylic acid esters included in the frictionreducing copolymers of the present invention are believed to increaseradii of gyration through steric repulsion, relative to those polymerscomposed entirely of acrylamide. Suitable methods used to determine theradius of gyration are well known by those skilled in the art. Examplesof acrylic acid esters suitable for use in the friction reducingcopolymers of the present invention include, but are not limited to,those acrylic esters shown in Table 1 with a high radius of gyration,such as 2-hydroxy-ethyl acrylate and 2-(2-hydroxy-ethoxy)-ethylacrylate. Those of ordinary skill in the art, with the benefit of thisdisclosure, will be able to select an appropriate acrylic acid ester toinclude in the friction reducing copolymers of the present inventionbased on a variety of factors, including the desired level of frictionreduction and flocculation properties

The amount of acrylamide and acrylic acid ester to include in thefriction reducing copolymers of the present invention may be determinedbased on a number of factors, including the desired friction reduction,flocculation properties, etc. Generally, the acrylamide may be presentin the friction reducing copolymers of the present invention in anamount in the range of from about 60% to about 95% by weight and anacrylic acid ester in an amount in the range of from about 5% to about40% by weight. In other embodiments, the acrylamide may be present inthe friction reducing copolymers of the present invention in an amountin the range of from about 70% to about 92.5% by weight and an acrylicacid ester in an amount in the range of from about 7.5% to about 30% byweight. In another embodiment, the acrylamide may be present in thefriction reducing copolymers of the present invention in an amount ofabout 85% by weight and an acrylic acid ester may be present in anamount of about 15% by weight.

Among other things, the friction reducing copolymers of the presentinvention should reduce energy losses due to friction in the aqueoustreatment fluids of the present invention. For example, the frictionreducing copolymers of the present invention may reduce energy lossesduring introduction of the aqueous treatment fluid into a well bore dueto friction between the aqueous treatment fluid in turbulent flow andthe formation and/or tubular good(s) (e.g., a pipe, coiled tubing, etc.)disposed in the well bore. Further, due to the presence of an acrylicacid ester, the friction reducing copolymers of the present inventionmay have a reduced sensitivity to salts as compared to copolymers usedpreviously for friction reduction, as well as a reduced ionic nature,thereby minimizing the flocculation properties thereof.

The friction reducing copolymers of the present invention should have amolecular weight sufficient to provide a desired level of frictionreduction. Generally, friction reducing copolymers having highermolecular weights may be needed to provide a desirable level of frictionreduction. For example, in some embodiments, the weight averagemolecular weight of the friction reducing copolymers may be in the rangeof from about 7,500,000 to about 20,000,000, as determined usingintrinsic viscosities. Those of ordinary skill in the art will recognizethat friction reducing copolymers having molecular weights outside thelisted range may still provide some degree of friction reduction in anaqueous treatment fluid.

The friction reducing copolymers of the present invention should beincluded in the aqueous treatment fluids of the present invention in anamount sufficient to provide the desired reduction of friction. In someembodiments, a friction reducing copolymer of the present invention maybe present in an amount in the range of from about 0.01% to about 4% byweight of the aqueous treatment fluid. In some embodiments, a frictionreducing copolymer of the present invention may be present in an amountin the range of from about 0.025% to about 0.2% by weight of the aqueoustreatment fluid.

The friction reducing copolymers suitable for use in the presentinvention may be made in accordance with any of a variety ofpolymerization methods. In one embodiment, a suitable friction reducingcopolymer may be prepared using an emulsion polymerization technique.Those of ordinary skill in the art, with the benefit of this disclosure,will recognize an appropriate polymerization method to synthesize asuitable friction reducing copolymer. The present invention does not liein the polymerization method used to synthesize the friction reducingcopolymers of the present invention so long as it yields the desiredfriction reducing copolymer.

After polymerization, the friction reducing copolymers of the presentinvention may be provided in any suitable form, including in a solidform, in an oil-external copolymer emulsion, or as a component of anaqueous solution. Preparation of an oil-external copolymer emulsion willbe described in more detail below.

Additional additives may be included in the aqueous treatment fluids ofthe present invention as deemed appropriate by one of ordinary skill inthe art, with the benefit of this disclosure. Examples of such additivesinclude, but are not limited to, corrosion inhibitors, proppantparticulates, acids, fluid loss control additives, surfactants,breakers, iron-control inhibitors, scale inhibitors, and claystabilizers. For example, an acid may be included in the aqueoustreatment fluids, among other things, for a matrix or fracture acidizingtreatment. In fracturing embodiments, proppant particulates may beincluded in the aqueous treatment fluids to prevent the fracture fromclosing when the hydraulic pressure is released.

II. Oil-External Copolymer Emulsions

In embodiments where a particular friction reducing copolymer of thepresent invention is an oil-external copolymer emulsion, theoil-external copolymer emulsion may comprise water, a water-immiscibleliquid, an emulsifier, and a friction reducing copolymer of the presentinvention. Suitable oil-external copolymer emulsions further maycomprise inhibitors, salts, and inverters.

The water present in the oil-external copolymer emulsions generallyincludes fresh water, salt water, brine, seawater, or combinationsthereof. Generally, the water used may be from any source, provided itdoes not contain an excess of compounds that may adversely affect othercomponents in the oil-external copolymer emulsion. In some embodiments,the water may be present in the oil-external copolymer emulsion in anamount in the range of from about 35% to about 50% by weight of theemulsion.

Suitable water-immiscible liquids may include, but are not limited to,water-immiscible solvents, such as paraffin hydrocarbons, napthenehydrocarbons, aromatic hydrocarbons, and mixtures thereof. The paraffinhydrocarbons may be saturated, linear, or branched paraffinhydrocarbons. Examples of suitable aromatic hydrocarbons include, butare not limited to, toluene and xylene. An example of a suitablewater-immiscible solvent comprising paraffin hydrocarbons and napthenehydrocarbons is “LPA®-210”, available from Sasol North America, Inc.,Houston, Tex. The water-immiscible liquid may be present in theoil-external copolymer emulsion in an amount sufficient to form a stableemulsion. In some embodiments, the water-immiscible liquid may bepresent in the oil-external copolymer emulsions in an amount in therange of from about 20% to about 30% by weight.

Emulsifiers should be present in the oil-external copolymer emulsion,among other things, to facilitate the formation of an oil-externalcopolymer emulsion. Examples of suitable emulsifiers include, but arenot limited to, ethoxylated nonionic surfactants, guerbet alcoholethoxylate, and mixtures thereof. An example of a suitable emulsifiercomprises a tall oil fatty acid diethanolamine, such as “AMADOL® 511”,available from Akzo Nobel Surface Chemistry, Chicago, Ill. Anotherexample of a suitable emulsifier comprises a polyoxyethylene (5)sorbitan monooleate, such as “TWEEN® 81,” available from Uniqema, NewCastle, Del. Another example of a suitable emulsifier comprises asorbitan monooleate, such as “ALKAMULS® SMO,” available from RhonePoulenc, Inc., Paris, France. The emulsifier should be present in anamount sufficient to provide the desired stable oil-external emulsion.In some embodiments, the emulsifier may be present in an amount in therange of from about 0.5% to about 2.5% by weight of the emulsion.

The friction-reducing copolymers of the present invention that may bepresent in the oil-external copolymer emulsions are described above. Thefriction reducing copolymer should be present in the oil-externalemulsion in an amount that does not undesirably impact the emulsion'sstability. In some embodiments, the friction reducing copolymer may bepresent in an amount in the range of from about 30% to about 35% byweight of the emulsion.

In some embodiments, the oil-external copolymer emulsions further maycomprise a salt. The salt may be present, among other things, to addstability to the emulsion and/or reduce the viscosity of the emulsion.Examples of suitable salts, include, but are not limited to, ammoniumchloride, potassium chloride, sodium chloride, ammonium sulfate, andmixtures thereof. In some embodiments, the salt may be present in theoil-external emulsions in an amount in the range of from about 0.5% toabout 2.5% by weight of the emulsion.

In some embodiments, the oil-external copolymer emulsions further maycomprise an inhibitor. Among other things, the inhibitor may be includedto prevent premature polymerization of the monomers prior to initiationof the emulsion polymerization reaction. As those of ordinary skill inthe art will appreciate, with the benefit of this disclosure, thecopolymer may have been synthesized using an emulsion polymerizationtechnique wherein the inhibitor acted to prevent prematurepolymerization. Examples of inhibitors suitable for use in the presentinvention include, but are not limited to, quinones. An example of asuitable inhibitor comprises a 4-methoxyphenol (MEHQ). The inhibitorshould be present in an amount sufficient to provide the desiredprevention of premature polymerization. In some embodiments, theinhibitor may be present in an amount in the range of from about 0.001%to about 0.1% by weight of the emulsion.

In some embodiments, the oil-external copolymer emulsions further maycomprise an inverter. Among other things, the inverter may facilitatethe inverting of the emulsion upon addition to the aqueous treatmentfluids of the present invention. As those of ordinary skill in the artwill appreciate, with the benefit of this disclosure, upon addition tothe aqueous treatment fluid, the emulsion should invert, releasing thecopolymer into the aqueous treatment fluid. Examples of suitableinverters include, but are not limited to, ethoxylated alcohols,nonionic surfactant with an HLB of from 12 to 14, and mixtures thereof.An example of a suitable inverter comprises an ethoxylated C12-C16alcohol, such as “SURFONIC® L24-7,” available from Huntsman PerformanceProducts, The Woodlands, Tex. The inverter should be present in anamount sufficient to provide the desired inversion of the emulsion uponcontact with the aqueous fluid present in the aqueous treatment fluid.In some embodiments, the inhibitor may be present in an amount in therange of from about 1% to about 5% by weight of the emulsion.

In some embodiments, emulsion polymerization may be used to prepare asuitable oil-external copolymer emulsion that comprises a frictionreducing copolymer of the present invention. Suitable emulsionpolymerization techniques may have a variety of different initiationtemperatures depending on, among other things, the amount and type ofinitiator used, the amount and type of monomers used, the amount andtype of inhibitor used, and a number of other factors known to those ofordinary skill in the art. In one embodiment, a suitable emulsionpolymerization technique may have an initiation temperature of about 25°C. Due to the exothermic nature of the polymerization reaction, themixture may be maintained at a higher temperature than the initiationtemperature during procession of the polymerization reaction, forexample, in the range of from about 37° C. to about 45° C.

A variety of different mixtures may be used to prepare an oil-externalcopolymer emulsion comprising a friction reducing copolymer of thepresent invention. Suitable mixtures may include acrylamide, an acrylicacid ester, water, a water-immiscible liquid, an initiator, and anemulsifier. Optionally, the mixture further may comprise an inhibitor,an activator to initiate polymerization at a lower temperature, and aninverter. Those of ordinary skill in the art, with the benefit of thisdisclosure, will know the amount and type of components to include inthe mixture based on a variety of factors, including the desiredmolecular weight and composition of the friction reducing copolymer andthe desired initiation temperature.

III. Methods of the Present Invention

The aqueous treatment fluids of the present invention may be used in anysubterranean treatment where the reduction of friction is desired. Suchsubterranean treatments include, but are not limited to, stimulationtreatments (e.g., fracturing treatments, acidizing treatments, fractureacidizing treatments), and completion operations. Those of ordinaryskill in the art, with the benefit of this disclosure, will be able torecognize a suitable subterranean treatment where friction reduction maybe desired.

In some embodiments, the present invention provides a method of treatinga portion of a subterranean formation comprising: providing an aqueoustreatment fluid of the present invention comprising an aqueous fluid anda friction reducing copolymer of the present invention that comprisesacrylamide in an amount in the range of from about 60% to about 90% byweight and an acrylic acid ester in an amount in the range of from about10% to about 40% by weight; and introducing the aqueous treatment fluidinto the portion of the subterranean formation. In some embodiments, theaqueous treatment fluid may be introduced into the portion of thesubterranean formation at a rate and pressure sufficient to create orenhance one or more fractures in the portion of the subterraneanformation. In some embodiments, the aqueous treatment fluid may beintroduced into the subterranean formation at a rate in the range offrom about 30 barrels per minutes (“bpm”) to about 250 bpm. In someembodiments, the rate may be in the range of from about 50 bpm to about175 bpm. The portion of the subterranean formation that the aqueoustreatment fluid is introduced into will vary dependent upon theparticular subterranean treatment. For example, the portion of thesubterranean formation may be a section of a well bore, for example, ina well bore cleanup operation. In the stimulation embodiments, theportion may be the portion of the subterranean formation to bestimulated.

The methods of the present invention further may comprise preparing theaqueous treatment fluid. Preparing the aqueous treatment fluid maycomprise providing the friction reducing copolymer and combining thefriction reducing copolymer with the aqueous fluid to from the aqueoustreatment fluid. The friction reducing copolymer may be provided in asolid form, suspended in an oil-external copolymer emulsion, or as acomponent of an aqueous solution.

To facilitate a better understanding of the present invention, thefollowing examples of certain aspects of some embodiments are given. Inno way should the following examples be read to limit, or define, thescope of the invention.

EXAMPLE 1

The radius of gyration for several friction reducing copolymers wascalculated to determine acrylic acid derivatives that may be suitablefor use in the friction reducing copolymers of the present invention.Using Materials Studio, version 4.0, published by Accelrys, Inc., theamorphous cell module constructed a periodic boundary condition with athree dimensional box with 100 water molecules, model copolymers, and asalt. The model copolymers consisted of 10 monomers total, 7 acrylamidemonomers and 3 acrylic acid derivative monomers. The acrylic acidderivative monomers utilized are represented by the following formula:CH₂CHCO₂R, wherein the designated R group is identified in Table 1below. In those instances wherein R is H, the model copolymer consistedonly of acrylamide and acrylic acid.

TABLE 1 Radius of Acrylic Acid Derivative, R % Salt Additive Gyration(CH₂)₁₁CH₃ 2% CaCl₂ 4.403121094 H 2% Tetramethyl 4.528692831 AmmoniumChloride H 2% CaCl₂ 4.537672844 H 2% Potassium Formate 4.592102264CH₂CH₂OH 2% CaCl₂ 4.742162218 H 2% CaCl₂ 4.876152927 H 2% CaCl₂4.9123214 H 2% CaCl₂ 4.916635426 (CH₂CH₂O)₂CH₂CH₂OH 2% CaCl₂ 4.940284829H 2% Potassium Acetate 4.952512972 H NONE 5.085112313 H 2% CaCl₂5.183655561 (CH₂CH₂O)₃CH₂CH₂OH 2% CaCl₂ 5.25659844 CH₂CH₂OCH₂CH₂OH 2%CaCl₂ 5.354284857 CH₂CH₂OCH₂CH₂OCH₂CH₃ 2% CaCl₂ 6.000544538

Therefore, as seen from the results above, it was determined that it isdesirable to include an acrylic acid ester in the copolymers of thepresent invention because they generally have a larger radius ofgyration in the presence of interfering salts. Among the esters tested,2-(2-hydroxy-ethoxy)-ethyl acrylate was calculated to have the largestradius of gyration even in the presence of polyvalent metal ions such ascalcium.

EXAMPLE 2

A friction reducing copolymer that comprised acrylamide and an acrylicacid ester was prepared by a solution polymerization technique inaccordance with the following procedure. The acrylic acid ester used was2-(2-hydroxy-ethoxy)-ethyl acrylate. First, both the acrylamide and theacrylic acid ester were purified. To purify the acrylamide, chloroformwas heated to boiling and then added to 50 grams (“g”) of acrylamide. Aminimum amount of chloroform was then added to achieve solution,approximately 600 mL. Once solution was achieved, the mixture was thenremoved from the heat, sealed with parafilm, wrapped in foil and allowedto slowly cool to room temperature. After cooling, the solution wasplaced in a freezer. After approximately 1 hour, the acrylamide wasfiltered by a vacuum with a Buchner funnel in a fume hood. Theacrylamide was protected from light and heat before being placed in thereaction vessel. To purify the 2-(2-hydroxy-ethoxy)-ethyl acrylate, itwas passed through an Aldrich column purchased for the removal of4-methoxyphenol (MEHQ).

After the acrylamide and 2-(2-hydroxy-ethoxy)-ethyl acrylate werepurified, 1200 milliliters (“mL”) of water were then purged. To purgethe water, it was stirred mechanically at 300 rpm and a glass dispersiontube was submerged beneath the surface of the water while nitrogen wasforced through for approximately 12 hours to remove dissolved oxygen.After the tube was raised to prevent foaming, the stirring was stoppedand 36 g of sodium dodecyl sulfate was added to the water. Next, 45.3 gof the purified acrylamide and 17.3 g of the purified2-(2-hydroxy-ethoxy)-ethyl acrylate were added to the solution. Thissolution was then stirred for 72 hours to equilibrate the micellesbefore commencement of the polymerization. Finally, 0.025 g of aninitiator, azobisisobutyronitrile (“AIBN”) was added and the solutionwas heated to 50° C. for 18 hours. At the end of 18 hours, the polymerwas decanted for friction reduction testing.

EXAMPLE 3

To evaluate the relative effectiveness of conventional friction reducingcopolymers and friction reducing copolymers of the present invention inthe presence of interfering salts, friction reduction tests wereperformed using aqueous treatment fluids comprising either fresh wateror a 0.5% CaCl₂ brine solution. For this series of tests, thecomposition of the aqueous treatment fluids comprising a frictionreducing copolymer of the present invention are given in Table 2 below.

TABLE 2 Component % by weight Copolymer comprising acrylamide andacrylic 4.8 acid ester sodium dodecyl sulfate 2.8 Water/Brine Solution92.4 Total 100.0

These friction reduction tests were performed using a Friction ReductionMeter (“FR Meter”) in accordance with the procedure listed below. The FRMeter was a closed loop pipeline apparatus designed to measure thepressure drop across a 5-foot section of a 12-foot commercial steel pipeand a 5-foot section of a 12-foot smooth Hastaloy C pipe. The commercialsteel pipe had an inner diameter of 0.619 inches with a wall roughnessof 0.00011 feet. The Hastaloy C pipe had an inner diameter of 0.534inches. The FR Meter consisted of a storage/mixing tank connected to afixed speed progressive cavity pump which pumped the test fluid througha magnetic flow meter then through the test pipes and a return line tothe storage/mixing tank.

For each test, either 10 liters of tap water (fresh water) or 10 litersof tap water with a 0.5% CaCl₂ (brine solution) were added to thestorage/mixing tank. Next, the pump was run to circulate the water foran amount of time sufficient to fill all the pipes with water. Once thepipes were filled with water, the pump was stopped. The data acquisitionsystem was started, and the pump was started after about an additional10 to 15 seconds. The data acquisition system measured the flow rate,tank temperature, and pressure drop across the 5-foot section of eachpipe. At about 1 minute into the test, approximately 1% of the frictionreducing copolymer was added to the storage/mixing tank. The test wasrun for a total of about 20 minutes, with flow rate, tank temperature,and pressure drop across each 5-foot section of pipe recorded atone-second intervals. The pump rate was about 30 liters per minute, or 3system volumes per minute. For the commercial steel pipe, the flow wasfully turbulent at a Reynolds Number of about 50,000.

The first minute of data that was collected prior to the addition of thefriction reducing copolymer was used to verify instrument readings andprovide a baseline of data with a known fluid. The pressure drop acrossthe 5-foot section of pipe for the water was calculated from the flowrate and pipe dimensions in accordance with the following formula:

${\Delta\; P_{water}} = \frac{\rho\; V^{2}{Lf}}{2\; g_{c}D_{h}}$wherein ΔP_(water) is the calculated pressure drop for the water, ρ isdensity, V is the velocity, L is length, g_(c) is the gravitationalconstant, and D_(h) is the pipe diameter. The variable f was calculatedin accordance with the formula below for turbulent flow.

$f = \left\{ {{- 2}\;{\log\left\lbrack {\frac{ɛ/d}{3.7} - {\frac{5.02}{N_{Re}}{\log\left( {\frac{ɛ/d}{3.7} + \frac{14.5}{N_{Re}}} \right)}}} \right\rbrack}} \right\}^{- 2}$wherein ε is pipe roughness, d is the pipe diameter, and N_(Re) is theReynold's Number (Shacham, M., Isr. Chem. Eng., 8, 7E (1976)).

Following the addition of the friction reducing copolymer to the tank,the measured pressure drop was compared to the calculated pressure dropfor the water to determine the % Friction Reduction (“% FR”) using thefollowing equation:

${\%\mspace{14mu}{FR}} = {1 - \frac{\Delta\; P_{measured}}{\Delta\; P_{water}}}$wherein ΔP_(water) is the calculated pressure drop for the water andΔP_(measured) is the measured pressure drop after introduction of thefriction reducing copolymer.

To determine the relative effectiveness of the friction reducingcopolymers of the present invention and conventional friction reducingcopolymers in brine solution, the following equation was used:

${\%\mspace{14mu}{Effective}} = \frac{\%\mspace{14mu}{FR}_{brine}}{\%\mspace{14mu}{FR}_{{fresh}\mspace{14mu}{water}}}$

The results of this series of test are shown in Table 3 below.

TABLE 3 Friction Reducing Copolymer (Type) Acrylamide/AcrylicAcrylamide/Acrylic Measured Parameter Acid acid ester % Effective at 4min.  6.88% 93.06% in brine solution % Effective at 9 min. 13.36% 90.15%in brine solution % Effective at 14 min. 12.64% 90.23% in brine solution% Effective at 19 min. 13.09% 92.72% in brine solution Average %Effective 11.49% 91.54% in brine solution

The percent effectiveness at various times may be used to compare therelative performance of the friction reducing copolymers of the presentinvention and conventional friction reducing copolymers in the presenceof interfering salts. A higher sum should represent better frictionreduction. The start time for these measurements was when the frictionreducing copolymer was added to the storage/mixing tank.

These examples thus show that a friction reducing copolymer of thepresent invention may provide friction reduction in the presence ofpolyvalent cations.

EXAMPLE 4

Emulsion tests were performed using friction reducing copolymers of thepresent invention to determine if the friction reducing copolymers ofthe present invention exhibited reduced flocculation tendencies incomparison to conventional friction reducers. A copolymer of acrylamideand acrylic acid was mixed with water, CaCl₂, and kerosene in thepresence of 1% by weight polymer. This conventional friction reducerformed an intractable emulsion that precipitated from solution. Next, acopolymer of the present invention comprising acrylamide and an acrylicacid ester was mixed with water, CaCl₂, and kerosene in the presence of1% by weight polymer. The emulsion cleanly separated in less than 30seconds. This example demonstrates that the friction reducing copolymersof the present invention have reduced flocculation tendencies.

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. While numerous changes may be made bythose skilled in the art, such changes are encompassed within the spiritof this invention as defined by the appended claims. Furthermore, nolimitations are intended to the details of construction or design hereinshown, other than as described in the claims below. It is thereforeevident that the particular illustrative embodiments disclosed above maybe altered or modified and all such variations are considered within thescope and spirit of the present invention. In particular, every range ofvalues (e.g., “from about a to about b,” or, equivalently, “fromapproximately a to b,” or, equivalently, “from approximately a-b”)disclosed herein is to be understood as referring to the power set (theset of all subsets) of the respective range of values. The terms in theclaims have their plain, ordinary meaning unless otherwise explicitlyand clearly defined by the patentee.

1. A method of treating a portion of a subterranean formation,comprising: providing an aqueous treatment fluid comprising an aqueousfluid, and a friction reducing copolymer that comprises acrylamide and2-(2-hydroxy-ethoxy)-ethyl acrylate; and introducing the aqueoustreatment fluid into the portion of the subterranean formation.
 2. Themethod of claim 1 wherein the friction reducing copolymer comprisesacrylamide in an amount in the range of from about 60% to about 95% byweight and 2-(2-hydroxy-ethoxy)-ethyl acrylate in an amount in the rangeof from about 5% to about 40% by weight.
 3. The method of claim 1wherein the aqueous treatment fluid is introduced into the portion ofthe subterranean formation at a rate sufficient to create or enhance oneor more fractures in the portion of the subterranean formation.
 4. Themethod of claim 1 wherein the friction reducing copolymer is present inthe aqueous treatment fluid in an amount sufficient to reduce frictionwithout forming a gel.
 5. The method of claim 1 wherein the aqueoustreatment fluid is introduced into the formation at rate in the range offrom about 30 barrels per minute to about 250 barrels per minute.
 6. Themethod of claim 1 further comprising the step of preparing the aqueoustreatment fluid, wherein the step of preparing the aqueous treatmentfluid comprises: providing the friction reducing copolymer; andcombining the friction reducing copolymer and the aqueous fluid to formthe aqueous treatment fluid.
 7. The method of claim 6 wherein the stepof providing the friction reducing copolymer comprises providing anoil-external copolymer emulsion that comprises water, a water-immiscibleliquid, an emulsifier, and the friction reducing copolymer.
 8. Themethod of 7 wherein the oil-external copolymer emulsion invertssubsequent to combination with the water, releasing the frictionreducing copolymer into the water with which the oil-external copolymeremulsion was combined.
 9. The method of claim 1 wherein the frictionreducing copolymer is present in an amount in the range of from about0.01% to about 1% by weight of the aqueous treatment fluid.
 10. A methodof treating a portion of a subterranean formation, comprising: providingan aqueous treatment fluid comprising an aqueous fluid, and a frictionreducing copolymer that comprises acrylamide in an amount in the rangeof from about 60% to about 95% by weight and 2-(2-hydroxy-ethoxy)-ethylacrylate in an amount in the range of from about 5% to about 40% byweight; and introducing the aqueous treatment fluid into the portion ofthe subterranean formation.
 11. The method of claim 10 wherein theaqueous treatment fluid is introduced into the portion of thesubterranean formation at a rate sufficient to create or enhance one ormore fractures in the portion of the subterranean formation.
 12. Themethod of claim 10 wherein the friction reducing copolymer is present inthe aqueous treatment fluid in an amount sufficient to reduce frictionwithout forming a gel.
 13. The method of claim 10 wherein the aqueoustreatment fluid is introduced into the formation at rate in the range offrom about 30 barrels per minute to about 250 barrels per minute. 14.The method of claim 10 wherein the friction reducing copolymer ispresent in an amount in the range of from about 0.01% to about 1% byweight of the aqueous treatment fluid.
 15. A method of fracturing asubterranean formation comprising: providing an aqueous treatment fluidcomprising water, and a friction reducing copolymer that comprisesacrylamide and 2-(2-hydroxy-ethoxy)-ethyl acrylate; and introducing theaqueous treatment fluid into the subterranean formation at or above apressure sufficient to create one or more fractures in the subterraneanformation.
 16. The method of claim 15 wherein the friction reducingcopolymer comprises acrylamide in an amount in the range of from about60% to about 95% by weight and 2-(2-hydroxy-ethoxy)-ethyl acrylate in anamount in the range of from about 5% to about 40% by weight.
 17. Themethod of claim 15 wherein the friction reducing copolymer is present inthe aqueous treatment fluid in an amount sufficient to reduce frictionwithout forming a gel.
 18. The method of claim 15 wherein the aqueoustreatment fluid is introduced into the formation at rate in the range offrom about 30 barrels per minute to about 250 barrels per minute. 19.The method of claim 15 further comprising the step of preparing theaqueous treatment fluid, wherein the step of preparing the aqueoustreatment fluid comprises: providing the friction reducing copolymer;and combining the friction reducing copolymer and the aqueous fluid toform the aqueous treatment fluid.
 20. The method of claim 15 wherein thefriction reducing copolymer is present in an amount in the range of fromabout 0.01% to about 1% by weight of the aqueous treatment fluid.